Disentangling climatic effects from biotic contingencies during extreme events: Chihuahuan desert responses to droughts and deluges
Extreme climatic events are increasing in frequency and intensity as a result of global warming. Although extreme events are often viewed as having negative impacts on ecosystem dynamics, recent studies in the Chihuahuan Desert show that a 5-year wet period led to an unexpected increase in grass production in desertified mesquite shrublands. This nonlinear increase in perennial grass production resulted from a sequence of events that led to greater grass production than expected (i.e., higher water use efficiency) based on precipitation alone. A similar nonlinear response did not occur during a multi-year drought or in individual wet years. Our objectives were to determine: (1) if this unusual grass response also occurred in other shrublands, (2) how long high water use efficiency persisted after the end of the wet period, and (3) the abiotic and biotic explanatory variables related to grass response in each of three climatic periods based on rainfall: no trend (1990-1999), drought (2000-2003), and wet period (2004-2008). We used long term data (1990-2012) of climate, biomass, and aboveground net primary production (ANPP) from three locations within each of three shrubland ecosystem types (honey mesquite, creosotebush, tarbush) in the northern Chihuahuan Desert to address these objectives.
Grass production was greater than expected based on precipitation for all three shrubland types during the wet period, and it took fewer consecutive wet years to reach this unusual production in mesquite and tarbush (3 years) than in creosotebush (5 years). This greater grass production and increase in water use efficiency were maintained two years beyond the end of the wet period in all shrubland types. Precipitation was only correlated with grass production in creosotebush shrublands in no-trend years. Grass production was very low in the drought for all shrubland types. During the wet period, biotic contingencies in the form of previous year’s biomass or production were more important than precipitation amount to current year’s grass production in all shrubland types. Our results show that wet periods need to be distinguished from periods with no trend or drought when developing relationships between grass production and potential explanatory variables. Disentangling climatic effects from biotic contingencies will be particularly important to predicting grass production in shrublands as the frequency and intensity of wet periods increase.